1. Field of Invention
The present invention relates to a spray nozzle having an improved tip and its method of manufacture for providing a repeatable performance in terms of droplet size and spatial droplet distribution. The invention is particularly suitable for the fine atomization of small amounts of liquids.
2. Background of the Invention
Spray nozzles are used to spray small amounts of liquids in various applications, such as medical nebulizers, chemical analysis of liquid samples, spray drying and coating medical devices.
Such spray nozzles generally comprise a body with a liquid line and a removable tip having a central orifice provided at the atomizing end, which extends to an inner section. The spray nozzle may also include one or more passages for an atomizing fluid, which may be expelled through an annular gap or gas annulus provided between the body orifice and the tip orifice to disintegrate the liquid. Spray nozzles using compressed gas to disintegrate a liquid are also referred to as twin-fluid nozzles.
Optimum atomization and particle transport efficiencies generally depends on the spatial characteristics of the spray plume and on the droplet size which, in turn, depends on the roundness of tip orifice and concentricity between the tip orifice and inner section. This is particularly true, when an atomizing gas is provided through a comparatively small annular gap, which often has a width on the order of only 20 to 250 micrometers.
However, the lack of concentricity between the inner section and orifice of the tip is a common problem of prior art spray nozzles having comparatively small orifices. FIG. 1 depicts an enlarged view of a tip of an exemplary spray nozzle having a conical inner section extending to an orifice. It can be seen that the orifice 16 of the tip 3 is not concentric with respect to the inner surface 72 of the tip. In this example there is an eccentricity of 0.04 mm between axis of inner section 71 and axis of tip orifice 70.
Inner section-orifice eccentricity may result from machining the orifice and inner section of the tip in different setups. A further problem associated with conventional atomizing devices are imperfections of the orifices in terms of roundness and surface quality. The orifices are generally manufactured using conventional manufacturing procedures such as drilling. For instance, drilling of small holes can lead to spiral marks and burrs and may require secondary procedures such as electropolishing which, in turn, may result in manufacturing tolerance variations and/or out-of-roundness of the orifice. When manufacturing a series of spray nozzles comprising comparatively small orifices using current machining procedures the reproducibility within a badge may therefore not be assured. In addition, with current spray nozzles there is a risk of misalignment during disassembling and reassembling of the nozzle resulting in poor concentricity of the body in relation to the tip orifice. Thus, devices, even of the same type, often will have different spray characteristics resulting from very minor variations in terms of concentricity of tip orifice and inner section, orifice roundness and surface quality. To visualize the effect of an eccentricity between inner section of tip and tip orifice on the spray characteristics of a twin-fluid nozzle, the velocity distribution of the fluid exiting the tip orifice has been simulated using Computational Fluid Dynamics (CFD) software. FIG. 2A is a scalar representation and FIG. 2B is a vector representation of the fluid exiting the annular gap. Despite the optimum roundness and equal width of the tip orifice, there is an inhomogeneous velocity distribution at the annular gap, which is caused by a small eccentricity between the inner section of the tip and the tip orifice.
The spray performance in terms of symmetric spatial droplet distribution and tight droplet size distribution of spray nozzles is closely related to the roundness and concentricity of tip orifice and inner section of tip. In case of twin-fluid atomizers, any imperfection and eccentricity between the axes of the liquid orifice and the tip can cause the flow of the atomizing gas to be cylindrically asymmetric with respect to the axis of the liquid exiting from the liquid orifice. Hence, inhomogeneous gas velocities within the annular gap, as illustrated in FIGS. 2A and 2B, will lead to nebulization by the atomizing gas that is different on different sides of the spray plume. Consequently, poor spray stability and droplets that are too large and polydisperse in size may result in poor reproducibility and often poor stability during operation which, in turn, may lead to coating defects or reduced sample analysis efficiency.